They could be perminante allies to you as you could be there suppior lord, I found that when i was Oda that Hidioyshi (spelling) was basically an underling to me and his land was essentially owned by me. Once i had taken all the other land it was game over you have unified Japan.

Then there are lords you have made an actuall alliance with, there is a negotiation card that lets you cancel an allaince. Of course you have to unlock it fisrt with one of your officers.